Pallet with improved joint construction

ABSTRACT

Pallet made of wood members comprising upper and lower decks separated by side stringers and held together by adhesively bonded miter joints.

United States Patent [151 3,682,110 [451 Aug. 8, 1972 Harrison I [54] PALLET WITH IMPROVED JOINT CONSTRUCTION [72] Inventor: George C. Harrison, Roseville,

Minn.

73 Assigne e: Minnesota Minlngand Manufacturing Company, St. Paul, Minn.

[221 Filed: May 26, 1909 211 Appl.No.: 827,746

521 user. .Q ..108/51 511 1111.0. ..B65d 19/18 [58] FieldofSearch ..l08/51-58; 207/2092, 20.92 c, 20.92 A, 189; 161/38, 36; 312/140, 204, 25s

[56] ReferencesClted UNITED STATES PATENTS 881,785. 3/1908 Flora ..312/2ssnx 3,074,772 1/ 1963 Kessler et al. ..312/204 3,132,910 5/1964 Vigne .......312/140X 3,437,392 4/1964 Hilfinger ..312/258 2,388,730 11/1945 Fallert ..108/57 2,748,046 5/1956 Works et a1 ..108/51 H X 2,873,935 2/ 1959 DePew et a1 ..108/51 3,03 1,047 4/ 1962 3,148,637 9/1964 3,204,583 9/1965 Nicholson ..108/51 3,251,322 5/1966 Downs et al. ..108/58 3,498,654 3/ 1970 Diaz et a1 ..287/ 189.36 H 2,060,906 11/1936 Snyder 161/38 Primary Examiner-Bobby R. Gay Assistant Examiner-Glenn O. Finch Attorney-Kinney, Alexander, Sell, Ste1dt& Delahunt [57] ABSTRACT Pallet made of wood members comprising upper and lower decks separated by side stringers and held together by adhesively bonded miter joints.

4 Claims, 10 Drawing Figures PATENTED M19 8 I973 3. 682, l l 0 sum 2 OF 2 ATTORNE Y5 PALLET WITH IMPROVED JOINT CONSTRUCTION FIELD OF INVENTION This invention relates to pallets made of wood and used in storing and shipping bags, boxes, and other types of laden, and which can be handled by fork lift trucks. In another aspect, it relates to an improved joint construction which can be used to hold together pieces of wood and other types of construction material. In another aspect, it relates to a method for the construction of a pallet.

BACKGROUND OF THE PRIOR ART Pallets are extensively used today for storing and transporting heavy goods packaged in bags, boxes, etc., and other type of laden. Such pallets are commonly made of wood, a construction material which is of relatively low cost and light weight, and comprise upper and lower deck members separated by side stringers, leaving openings on the two ends to receive the forks of a lift truck and facilitate the handling or moving of the pallets.

Wooden pallets are subjected to a lot of dropping, jarring, and other abuse during their lifetime and a host of improvements have been described in the art as a means for prolonging the life and durability of such pallets, such art being typified by U. S. Pat. Nos. 2,178,646, 2,822,882, 2,873,935 and 2,982,507. One primary source of abuse which these improvements, and the subject invention, seek to prevent or minimize, is that due to the striking of the fork of a lift truck on the ends or sides of the pallets, which commonly results in splintering, abrasion, and weakening, or even collapsing, of the pallet members or joints.

Pallets are commonly subjected to the elements, which tend to warp or otherwise deform the wood members or lumber. The lumber used is often green and the exposed ends dry faster and checking results causing loss of rigidity, strength, and permanence. Overloading of the pallets, or unbalanced loading, also takes its toll.

Nails and screws commonly used to fasten' the pallet members together often split the wood and pull out slightly due to warping or twisting of the lumber, thus weakening the pallet. The heads of these fastening devices protrude from the pallet and catch on the goods, with consequent damage. Such mechanical fastening devices eventually work loose and fall out, causing further weakening and even collapse of the pallet.

Attempts to overcome the disadvantages of wooden pallets by using metal pallets have not been successful due to the higher cost and heavier weight of such pallets. So, even though wooden pallets are relatively short-lived, they do enjoy wide-spread use, and there is a need for an improved wooden pallet not subject to the disadvantages now commonly associated with wooden pallets.

SUMMARY OF INVENTION Briefly, the pallets of this invention are made of wooden members comprising upper and lower rectangular decks, fastened to and spaced apart by parallel side stringers, with a parallel stringer preferably running down the center between and attached to the decks. Each of the two side stringers is attached at its inner face to a supplemental stringer disposed between the two decks. These wooden members can be made of any type of lumber, but most advantageously are made out of plywood. A conventional five-ply sheet (4 feet X 8 feet or 4 feet X 10 feet,5/ inch to 1 inch thick) can be sawed to provide all the members used in this invention to assemble a single pallet of standard size. The mating edges of the two decks and side stringers are sawed at an angle, preferably 45, so as to provide miter joints which can be adhesively bonded together, preferably using for this purpose a longitudinal folded two-ply width of fabric saturated with a cure-in-place adhesive such as the adhesive described in U. S. Pat. Nos. 3,201,136, 3,251,322, and 3,422,589. A pallet constructed in this manner has joints which completely hide or cover the end grain of the mating portions of the pallet members. When the pallet members are made of plywood, the end grain of each ply of one joined member is adhesively bonded to end grain or with the grain of a ply of the other joined member. By completely hiding the end grain in this manner, delarnination of the structure due to stress is prevented or substantially minimized.

DRAWING In the accompanying drawing;

FIG. 1A is a side view of a conventional sheet of P y FIG. 1B is a similar side view showing how a sheet of plywood can be sawed to form the members used in assembling the pallet;

FIG. 1C is a side view illustrating how the pallet members can be aligned in a plane, ready for assembly;

FIG. 1D is an enlarged side view of the aligned pallet members in the process of being assembled;

FIG. 2 is an enlarged side view of a portion of FIG. 1D, showing details of the miter joint of this invention in the process of being assembled;

FIG. 3 is similar to FIG. 2 showing the miter joint as assembled;

FIG. 4 is a perspective view of the assembled pallet as it is being engaged by the fork of a lift truck;

FIGS. 5 and 6 are side views of different embodiments of the pallet of this invention; and

FIG. 7 is a diagramatic view of apparatus which can be used in making the adhesive saturated web of fabric preferably used in adhesively bonding the miter joints of this invention.

DETAILED DESCRIPTION OF THE INVENTION As mentioned above, the pallet of this invention advantageously can be formed from a single sheet of plywood of conventional size. FIG. 1A shows a side view of a piece of such plywood and FIG. 1B shows how it can be sawed across its width to provide the pallet members, these pallet members comprising an upper or loading platform or deck member 2 and a lower platform or deck member 3, supporting side stringers 4 and 5, and supplemental supporting stringers 6, 7 and 8. Note that deck members 2 and 3 and side stringers 4 and 5 are sawed at an angle, preferably 45, while the supplemental stringers 6, 7 and 8 are sawed in a plane normal or perpendicular to their face.

The only waste resulting from sawing the sheet of plywood in this manner is that designated by reference numbers 9 and 10. In sawing a conventional 4 feet X 8 feet sheet of plywood in this manner, all of the members will of course be 4 feet in length; each deck 2 and 3 will have an outer face 40 inch wide and an inner face 39 inch wide. Each of the two side stringers 4 and 5 will have an outer face 4 inch wide and an inner face 3 inch wide, and supplemental stringers 6, 7 and 8 will have 3 inch widths.

The pallet members are then arranged or aligned in a plane, e.g. on the floor, in the manner shown in FIG. 1C, the side stringers 4 and 5 being turned upside down for this purpose. Tapes 11, coated with pressure sensitive adhesive, are placed on the bottom of the assembly to cover the joint lines of the abutting decks and side stringers. The outer side edges of side stringer 4 abuts adjacent outer sideedges of decks 2 and 3 and forms a groove, and an outer side edge of side stringer 5 abuts an adjacent outer side edge of deck 3 and also forms a groove. Supplemental side stringers 6 and 7 are disposed so as to lie on top of the so-disposed side stringers 4 and 5, respectively, and are suitably fastened thereto, for example by a plurality of staples 12 or the like. The next step of assembly comprises placing a longitudinally-folded, two-ply web of fabric 13, saturated with a cure-in-place adhesive, in each of the grooves formed by the abutting edges of the decks and side stringers, and placing a similar web 13 on the beveled end left on either side of the aligned pallet members. The center supplemental stringer 8 is placed perpendicular on the top of the inner face of deck 3, with a folded, adhesive-saturated web 15 disposed therebetween and a like web 15 placed on the top side of stringer 8, the latter being disposed in the center of deck 3 and running parallel to the sides of the decks. The aligned pallet members are then simply assembled by lifting up one edge of the assembly and bringing it over to mate with the lifted stringer on the other side of the assembly, as shown by FIG. 1D, the tapes 11 serving as temporary hinges for this purpose. FIGS. 2 and 3 show in detail how the mating joint portions of the assembly are brought together, these enlarged views also showing the individual plies of the plywood used in forming the pallet members and the plies making up the web of adhesively saturated web. As shown in FIG. 3, some of the adhesive 14 is eventually squeezed out from the web 13 so as to provide a layer of adhesive between the abutting edges of the supplemental stringers and the adjacent inner face of the deck. Following assembly, the pallet can be stacked and set aside to allow the adhesive to cure at room temperature and tapes 11 can be pulled off if desired. After assembly and during cure, the weight of the top deck is supported by the supplemental stringers, eliminating or minimizing the tendency of the deck to dislodge the side stringer and adhesive-saturated web.

Referring now to FIG. 4, this perspective view of the finished pallet of this invention illustrates how the forks 16 of a lift truck generally designated 17 can be inserted into the open ends 18 of the pallet. Further rigidity of the pallet is preferably provided by running one or more additional supplemental parallel stringers 19 down the center of the pallet between the two decks 2 and 3. Such additional stringers can be stapled or otherwise secured to center stringer 8 and can be positioned at the same time as the latter while the components are aligned on the floor before final assembly.

The pallet described above is made out of decks which are each made of a single piece of plywood. FIGS. 5 and 6 illustrate other embodiments of this invention where the upper and lower decks are made out of slats or relatively narrow strips of plywood or the like which are spaced apart and run perpendicular to the side stringers. The number and spacing of slats on the upper deck will be determined by the particular use to be made of the pallet and the economies desired by using less lumber for such purposes. The number and spacing of the slats making up the lower deck of such pallet modifications will be similarly determined. Additionally, if the particular lift truck used is one which has wheels fastened at the end of the forks, the spacing of the slats making up the lower deck can be arranged so as to accommodate such wheels, permitting them to be lowered through the spacings between the slats, as shown in FIG. 5. In any case, the ends of the slats are cut on an angle and they are joined to the side stringers and supplemental stringers in the manner shown in FIGS. 2 and 3, except that instead of using an adhesively-saturated web, a simple coating of the cure-in-place adhesive can be applied, for example by brushing, on the edge of the side and supplemental stringers to be assembled.

In the preferred embodiment of this invention, the miter joint between the decks and side stringers is preferably accomplished by using a two-ply web of fabric saturated with a cure-in-place adhesive. The fabric is preferably loosely woven, e.g. with 50 percent or more open space, and made of polyester, vinyl, or polyethylene fiber. In FIG. 7, I have illustrated apparatus which can be used to make such adhesive web. Referring now to that figure, plies 13a and 13b, from suitable sources, travel downwardly over suitable rollers 21 and are pinched together by another pair of rollers 22. A supply 14 of the cure-in-place adhesive is ejected from a suitable hopper 23 or the like, for example by means of plunger 24 into the small clearance between the two plies 13a and 13b, the resulting saturated web 13 being cut to suitable length and then preferably longitudinally folded and placed in the grooves formed by the abutting edges of the side stringers and decks.

As mentioned, I prefer to use the two-part polyurethane adhesive or bonding substance described in U. S. Pat. Nos. 3,201,136 and 3,251,322, such adhesive being a polyalkylene ether glycol-based, preferably amine-modified, crosslinked polyurethane elastomer or rubber, which cures from its liquid state to a solid state at normal room temperature. The resulting cured joint is permanently flexible and water-proof and had high strength, and has the ability to dampen the various impact forces that the pallet normally receives when handled by mechanical equipment. Such adhesive system is especially efficient in bonding the ends of plywood together, that is, it is efficient at end-grain gluing. Unlike most adhesives, such adhesive does not soak into the end-grain and become deficient at the glue line, nor does it shrink on curing or become honeycombed or deficient on loss of solvent. The adhesive system appears to chemically react with the wood to effect a chemical as well as an adhesive bond.

An example of a suitable two-part polyurethane formulation is as follows:

PART A Parts by weight Polypropylene ether glycol (2000 MW.) 322.0 4,4'-methylene bis(2-chloroaniline) 42.0 Catalyst (usually an organo-metallic compound or metal organic salt or mixture thereof, e. g., phenyl mercuric acetate) 2.4 Fillers, preservatives, and other additives 33.6

Part B Parts by weight Toluene diisocyanate 62,3 Polypropylene ether glycol (400 MW.) 3L4 Polypropylene ether triol (400 M.W 6.3

Alternatively, other types of wood-to-wood adhesives can be used, such as polyester resins, epoxy resins, rubber-phenolic compositions, and the like, this invention not being limited to the aforementioned preferred adhesive.

The adhesively-bonded miter joint of this invention can be used in general construction work, though I have found it particularly applicable in the construction of pallets. As shown clearly by FIG. 3, this manner of bonding completely covers and bonds the end-grain of the wood, leaving no end-grain exposed and liable to checking, splitting, etc. Further, this type of bonded joint is stronger than joints resulting when the face of a piece of plywood is bonded to the face or end-grain of another piece of plywood, application of tensile or stress to the latter type of joint often resulting in the lifting of the ply or delamination just below the glue line, the cohesive strength between the outer and the next inner ply being a limiting factor which is ordinarily subject to further weakening due to the exposure of the end-grain which is liable to checking.

Various modifications and alterations of this invention will become apparent to those skilled in the art without departing from the scope and spirit of this invention, and it should be understood that this invention is not to be unduly limited to the illustrative embodiment set forth herein.

I claim:

1. A wooden pallet comprising upper and lower rectangular decks made of plywood cut so as to provide beveled end-grain edges, two side stringers made of plywood cut so as to provide beveled end-grain edges, said side stringers being disposed between said decks at their edges and holding them in spaced-apart, parallel relation, the mating beveled edges of said decks and stringers being in the form of an adhesively-bonded miter joint, and supplemental stringers made of plywood, each of said supplemental stringers being adhesively bonded at its edges to the adjacent inner faces of said decks.

2. A pallet according to claim 8 wherein said adhesively bonded miter joint further comprises a folded web of fabric impregnated with a cure-in-place adhesive.

3. The construction of claim 1, wherein said adhesive is a tough, shock-absorbent, crosslinked elastomer strongly adherent to wood and comprises polyalkylene ether glycol based polyurethane reaction product of a liquid reaction mixture solidified in the presence of a low temperature curing catalyst.

4. A pallet according to claim 1 wherein said supplemental stringers comprise two supplemental stringers,

each of which is fastened to the inner face of a said side stringer, and a center stringer of plywood disposed between said decks parallel to said side stringers. 

1. A wooden pallet comprising upper and lower rectangular decks made of plywood cut so as to provide beveled end-grain edges, two side stringers made of plywood cut so as to provide beveled endgrain edges, said side stringers being disposed between said decks at their edges and holding them in spaced-apart, parallel relation, the mating beveled edges of said decks and stringers being in the form of an adhesively-bonded miter joint, supplemental stringers made of plywood, each of said supplemental stringers being adhesively bonded at its edges to the adjacent inner faces of said decks.
 2. A pallet according to claim 8 wherein said adhesively bonded miter joint further comprises a folded web of fabric impregnated with a cure-in-place adhesive.
 3. The construction of claim 1, wherein said adhesive is a tough, shock-absorbent, crosslinked elastomer strongly adherent to wood and comprises polyalkylene ether glycol based polyurethane reaction product of a liquid reaction mixture solidified in the presence of a low temperature curing catalyst.
 4. A pallet according to claim 1 wherein said supplemental stringers comprise two supplemental stringers, each of which is fastened to the inner face of a said side stringer and a center stringer of plywood disposed between said decks parallel to said side stringers. 